A wireless communication system defines different channels over which to transmit data between a radio network node and a wireless communication device. Different types of channels carry different types of data. For example, a data channel carries user data, and a control channel carries control data. Because the control data supports transmission of the user data over the data channel, the control data proves important enough to transmit with as low a modulation order as necessary to guarantee robustness against noise and interference.
In at least some contexts, though, practical constraints prevent the system from dynamically adapting the control channel's modulation order to current channel conditions. This effectively requires the system to statically fix the control channel's modulation order to the most robust option, and thereby artificially cap the control channel's spectral efficiency even in the presence of favorable channel conditions. With the control channel's spectral efficiency limited in this way, the control channel requires more transmission resources than would otherwise be required.
Consider for instance a Physical Downlink Control Channel (PDCCH) in a system that is or evolves from Long Term Evolution (LTE). A PDCCH is a physical channel that carries control data called Downlink Control Information (DCI). This DCI among other things supports transmission of user data over the Physical Downlink Shared Channel (PDSCH). The DCI in this regard informs a user equipment (UE) about the location of the physical resource block pairs allocated to it in the PDSCH, as well as the modulation scheme that the UE needs to use for recovering its user data on the PDSCH. Although the PDCCH signals the PDSCH's modulation order, there is no mechanism to signal the modulation order of the PDCCH itself. And requiring the UE to blindly detect that modulation order would unacceptably increase UE complexity. As a result, the PDCCH's modulation scheme remains fixed as Quadrature Phase-Shift Keying (QPSK). With the PDCCH's modulation order thereby fixed at the most robust order of 4, the maximum spectral efficiency for the PDCCH is always less than 2 bits/sec/Hz, even for UEs with the most favorable channel conditions. Similar challenges exist for the enhanced PDCCH (ePDCCH) as another example.